The invention relates to a machine for testing elements having magnetic bubbles, such as memories, as well as to a testing method using such a machine.
It is pointed out that conventionally the term "bubbles" is used to describe generally cylindrical magnetic domains created in a planar magnetic film defining a monocrystalline layer having a uniaxial magnetic anisotropy perpendicular to the plane of the layer. The magnetic induction of these cylindrical domains has a direction opposite to that which it has in the remainder of the layer. These magnetic domains have the special feature of being displaceable in the layer when the latter is coated with magnetizable metallic designs or patterns (for example of permaloy) ensuring the guidance of the magnetic domain. The stability of the bubbles results from the application of a polarization field parallel to the magnetization direction of the magnetic film, i.e. perpendicular to the plane of said film. The displacement of these domains is obtained by means of a rotary field parallel to said plane by means of which the metallic patterns formed on the magnetic film are magnetized in such a way that the magnetic domains are positioned at the point where the vertical component of the stray field is at a maximum and of opposite direction to the polarization field and can be displaced by rotating said field in such a way as to modify the magnetic poles formed on the magnetized metallic patterns.
In this way it is possible to produce logic circuits, comparators, memories, etc.
As in all electronic components elements having magnetic bubbles must be tested after manufacture so that it is possible to check their performances and detect possible manufacturing faults.
A machine for testing components with magnetic bubbles must have a magnetic head making it possible to create the polarization field and rotary field necessary for the operation of the element to be tested, as well as an actual test device permitting the application of voltages and the circulation of the necessary currents, whilst also making it possible to carry out the necessary measurements and observations.
At present several different machines are known for testing elements having magnetic bubbles of this type. However, none of these machines permits the continuous testing of elements with magnetic bubbles of medium or large size, due to the often contradictory conditions imposed by the magnetic characteristics of the magnetic head and by the mechanical constraints linked more particularly with the accessibility of the tested magnetic film and arrangement of the test device with respect to the magnetic head.
Thus, the magnetic head must simultaneously supply a continuous polarization field normal to the plane of the magnetic film between 100 and 300 oe and a rotary field located in the plane of the magnetic field and of frequency between 1 and 200 kHz, and amplitude between 10 and 70 oe.
The creation of the polarization field causes few problems and can easily be attained by means of two coils arranged in the Helmholtz position. However, it is more difficult to create the rotary field. Thus, the rotary field is created by means of two pairs of coils energized by two sinusoidal currents dephased by .pi./2 and which cannot be placed in the Helmholtz position. This makes it impossible to create a homogeneous rotary field in the volume of the magnetic film to be tested. Moreover, the choke of the coil supplying the rotary field increases in size and importance in proportion to the frequency of the said field, which very rapidly limits the size of the coils or the working frequency. Furthermore the rotary field supplied by the coils has, in addition to the component located in the plane of the magnetic film, a component normal to said plane which disturbs the operation of the element to be tested.
Due to the non-homogeneous nature of the rotary field supplied by the magnetic head, in order to take account of the difficulties inherent in the creation of the rotary field and to reconcile them with the resulting mechanical consequences with regard to the position of the magnetic head coils relative to the film to be tested, the known testing machines are limited to the testing of small magnetic films and do not always make it possible to carry out continuous tests.
Thus, the presently available machines generally require the prior cutting of layers with a large surface area, called wafers, into small portions each carrying an elementary device called chips. Each chip which is to be placed in a field application box having more particularly cylindrical shielding and permanent magnets must then be equipped with control and/or output conductors before being placed in the "test box" provided with sets of polarization coils and of rotary field suitable for placing the same under operating conditions, whilst the conductors are connected to the test electronics. Thus, all the operations corresponding to a complete test are performed on each good or bad chip.